During his reign the capital of Japan was moved from Heijo, near Nara, first to Nagaoka, and then to Heian. This marks the beginning of the Heian era in Japanese history.

Kammu also sponsored the travels of the monks Saicho and Kukai to China, from where they returned to found the Japanese branches of, respectively, Tendai and ShingonBuddhism. He was an active emperor who set up new government organisations and fought the Ezo tribes in the north of the country.